Why can the USS Constitution be called a national ship?

Why can the USS Constitution be called a national ship?
USS Constitution was one of the first six ships built for the new United States Navy in 1797. Constitution, a wooden ship, is often called “Old Ironsides,” a nickname she got in battle during the War of 1812 when sailors watched in amazement as cannonballs bounced off her thick oak sides.

Reference:
USS Constitution's Crew - National Maritime Historical Society
Aug 5, 2019

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